Generally layoffs involve someone who doesn't know who you are picking names almost at random from a spreadsheet. Management may fight for certain people to stay. Then legal and HR get involved and look through the layoff list to see if the chosen employees are problematic. For example, if the layoffs include too many people from protected classes, which opens them up to being sued. For example, if your company is 20% women but the layoffs are 50% women, that's going to be an issue.

Avoiding paying substantial retention bonuses can work the same way, if a pattern can be shown.

A simple letter from a lawyer probably won't do anything. Large companies are prepared for that.

For anyone who does come across this, here's my best advice: if you are acquired and your new employment contract includes a retention bonus, you want that contract to say that the retention bonus is payable unless:

1. You leave voluntarily within that period; or

2. You are terminated with cause within that period.

Otherwise, you should get it.